This invention relates to a chemical stripper composition that protects copper metal during the removal of cross-linked polymer coatings using conditions of acidic conditions (low pKa). Materials removed in low pKa conditions include negative-tone chemically amplified (e.g. epoxy) and acid-catalyzed photoimageable coatings. Many commercialized strippers for microelectronic coatings do not perform sufficiently to meet minimum manufacturing requirements. The invention provides a commercial framework to create removal products for cross-linked systems that respond in acidic media without the harmful etching and damaging effects commonly observed on devices that contain copper.
For various processed conditions up to and including hard-baking or otherwise referred to as full-cure, the composition will remove and dissolve chemically-amplified reacted compounds within minutes without damaging effects to sensitive metals such as copper, using conventional immersion conditions at elevated temperatures. Such full-cure coatings are found to be resistant to conventional organic strippers that commonly comprise alkaline ingredients as exemplified in U.S. Pat. No. 6,551,973 (2003), Moore et al. Using these conventional strippers, no dissolution occurs. Instead, these conventional alkaline strippers are observed to remove the coating by mechanisms of lifting or breaking-up into pieces. This lift-off mechanism generates incomplete removal from complex three dimensional topographies as commonly seen in microelectromechanical systems (MEMS) devices. Un-dissolved material will produce particles that are circulated throughout the bath, causing re-deposition of the un-dissolved pieces onto other areas of the device. Such contamination that occurs onto these tiny computer controlled gears, sensors, springs, pumps, and related micro or nano-scale fixtures results in contamination and device failure. It is an object of this invention to achieve full-dissolving of the unwanted polymer material during the given stripping and removal period.
Full dissolution offers the advantage of efficient rinsing and filtration of the recycled composition. The composition has been found to be especially useful in the manufacture of semiconductor wafers, MEMS devices, and displays. During the manufacture of these microcircuits or micro-devices, various inorganic substrates such as single and polycrystalline silicon, hybrid semiconductors such as gallium arsenide, and metals, are coated with an organic coating (“photoresist”, or resist) which forms a resistant framework of permanent or temporary design and exhibits a pattern after undergoing a photolithographic process. The resist may be utilized to insulate conductors or protect selected areas of the substrate surface, such as silicon, silicon dioxide, or aluminum, from the action of chemicals in both wet (chemical) and dry (plasma) forms. In the case of the material being utilized as a photoresist, exposed areas of the substrate may carry out a desired etch (removal) or deposition (addition) process. Following completion of this operation and after subsequent rinsing or conditioning, it is necessary that the resist and any application post-etch residue be removed to permit essential finishing operations. Upon removal of the resist, specific micro-etched or deposited patterns are left behind. The masking and patterning processes are repeated several times to produce layered arrangements that comprise the art of the final device. Each step requires complete resist stripping and dissolving, to ensure that the final form device is produced at relatively high yields and performs satisfactorily. In US Application No. 2014/0076356 (2012), Dariot et. al describe compositions that use ether solvents with acidic additives to remove certain cross-linked polymer systems. Additionally, in the abandoned application as US No. 2011/0253171A1 (2011), Moore describes stripping chemistries that also acidic additives effect the removal of epoxy-based photoimageable coatings. Neither of these applications teach about chemistries that sufficiently protect copper surfaces to the extent necessary for microelectronic production. More specifically, this application from Dariot et. al is incomplete and not sufficient to ensure metal safety. Namely, there is one example citing a formula that contains oxalic acid, a compound known to produce undesirable residue. Oxalic acid is a common industrial carboxylic acid used for cleaning and complexing oxides of iron and other metals, however, it has strong precipitating effects with due to the copper-oxalate solubility product (Ksp) of 10{circumflex over ( )}e-22. Residue from this complex, while it may be acceptable for qualitative analysis in grade school, is difficult at best for scaling to a manufacturing process. Additionally, the application by Moore et. al describes the use of inhibitors benzotriazole (BTA) and tolytriazole (TTA), however, these are known to not perform in acidic media. While both of these applications teach removal and stripping practices in acidic media (low pKa), their suggestions for metal protection are insufficient for use in manufacturing practices. It is a further object of this invention to provide stripping and removal practices that can protect complex copper substrates.
It is an object of this invention to provide an improved stripping composition that will remove negative-tone PAG and chemically amplified photoresist that and achieve complete dissolution in minutes. It is also an object of this invention to conduct such photoresist removal from substrates without attack to underlying exposed copper as well as other metals. It is a further object to conduct this photoresist removal and metal protection by utilizing a safe and non-regulated chemistry that does not produce harm to workers or to the environment. These objects and others shall be conducted upon complex inorganic substrates used in the manufacturing of micro-devices such as semiconductors, MEMS, and displays.